That is the question asked--and answered--by the London Times, which has produced a Top 20 for readers to debate. And the list is pretty good!
20-16: Seven--Blair Witch--Memento--Planet of the Apes (original)--Shawshank Redemption
15-11: Gone With the Wind--Dr. Strangelove--Les Diaboliques--Wizard of Oz--Thelma & Louise
10-6: Sixth Sense--Usual Suspects--Italian Job (original)--Some Like it Hot--Breakfast at Tiffany's
2. Butch Cassidy
The ending of Carrie is awfully scary the first time you see it, but does not resonate much beyond the initial fright. And Butch Cassidy at #2 is a stretch. Like Thelma and Louise at #11, it is certainly memorable and iconic, but on repeat viewing, neither movie holds up very well, and both conclusions appear trite and even cheesy. Casablanca is an obvious choice, but no less correct for that; the same is true of Chinatown, which should probably be in the #1 spot.
The rest of the picks are strong, especially Sixth Sense, Usual Suspects, Les Diaboliques, Shawshank, and the eternally underrated (or at least underviewed) Memento, the last ten seconds of which manage to give the whole film another layer and get your heart racing.
Still, where is It's A Wonderful Life? And what about both Godfather movies, particularly the second one (is there a more underrappreciated scene in film than the brilliant flashback coda)? Silence of the Lambs and The French Connection perhaps deserve a mention, as do To Kill a Mockingbird, The Searchers, White Heat and High Noon. But what do readers think?
Update: See Alex Massie for more.